Nh. Damrauer et Jk. Mccusker, Ultrafast dynamics in the metal-to-ligand charge transfer excited-state evolution of [Ru(4,4 '-diphenyl-2,2 '-bipyridine)(3)](2+), J PHYS CH A, 103(42), 1999, pp. 8440-8446
The transition metal complexes [Ru(dmb)(3)](2+) and [Ru(dpb)(3)](2+), where
dmb is 4,4'-dimethyl-2,2'-bipyridine and dpb is 4,4'-diphenyl-2,2'-bipyrid
ine, have been studied by femtosecond visible electronic absorption spectro
scopy. Spectroelectrochemical measurements in conjunction with nanosecond t
ime-resolved absorption spectroscopy allow for the assignment of various fe
atures in the excited-state differential absorption spectra as both ligand-
based pi* <-- pi* and ligand-to-metal charge transfer (LMCT) in nature. A u
nique absorptive feature centered at similar to 530 nm in [Ru(dpb)(3)](2+)
was identified as an optical marker for the thermalized (and hence fully in
traligand delocalized) excited state. Single wavelength and full spectrum t
ransient absorption data were obtained on both molecules in CH3CN solution
at room temperature following metal-to-ligand charge transfer (MLCT) excita
tion at 400 nm. Data on [Ru(dmb)(3)](2+) at 532 nm, a region of net excited
-state absorption, revealed biphasic decay kinetics (similar to 120 fs and
5 ps) attributed to a combination of (MLCT)-M-1 --> (MLCT)-M-3 intersystem
crossing and vibrational cooling dynamics. Dynamics for [Ru(dpb)(3)](2+) un
der identical conditions revealed biphasic rise times in the region of the
ligand-based pi* <-- pi* absorption at lambda(probe) = 532 nm. Although the
origin of the fast component (similar to 200 fs) is not yet clear, the ca.
2 ps rise is assigned to rotation of the peripheral aryl ring and thus cor
responds to the time scale for intraligand electron delocalization.